A known superconducting apparatus includes a superconducting coil generating a magnetic flux when an electric power is supplied to the superconducting coil. In particular, a superconducting apparatus disclosed in JP2007-89345A (hereinafter referred to as Patent Document 1) includes a vacuum container forming a vacuum heat insulation chamber accommodating a superconducting coil. When an electric power is supplied to the superconducting coil, the superconducting coil generates a magnetic flux. The generated magnetic flux penetrates through the vacuum container. In this case, when the magnetic flux varies, an eddy current occurs in accordance with the electromagnetic induction and flows through the vacuum container in a direction that the variations of the magnetic flux are prevented. Accordingly, the vacuum container may be heated. In order to prevent the vacuum container from being heated, the vacuum container is made of a nonmetallic material such as a resin, a composite resin reinforced by a reinforcing material, and ceramic, which are having a high electric resistance. For example, the reinforcing material is a glass fiber and the ceramic is an alumina material.
According to the superconducting apparatus disclosed in Patent Document 1, heating of the vacuum container by the eddy current is prevented. However, since a base material of the vacuum container is formed by the above-mentioned nonmetallic material, heat due to thermal radiation may be transmitted to the vacuum container. This is because emissivity and absorption of the nonmetallic material due to thermal radiation do not largely differ from those of a metal material under a condition where an electromagnetic wave generated by the variations of the magnetic flux is within a visible wavelength range. Meanwhile, the emissivity and absorption of the nonmetallic material is extremely larger than those of the metal material under a condition where the electromagnetic wave is in an infrared wavelength range.
In particular, in the case of the vacuum container accommodating the superconducting coil, a temperature difference between a heat radiation side, which is at high temperatures, and a heat absorption side, which is at low temperatures, is very large. In general, emissivity and absorption of thermal radiation is basically proportional to a difference between the forth power of the absolute temperature of an object at high temperatures and the fourth power of the absolute temperature of an object at low temperatures. Accordingly, the superconducting coil tends to be heated under the influence of such emissivity and absorption.
A need thus exists for a superconducting apparatus and a vacuum container for the same, which are not susceptible to the drawback mentioned above.